380. Insert Delete GetRandom O(1) 🎲

Difficulty: Medium - Tags: Design, Hash Table, Randomized

Description

Implement the RandomizedSet class:

• RandomizedSet() Initializes the RandomizedSet object.

• bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.

• bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.

• int getRandom() Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.

You must implement the functions of the class such that each function works in average O(1) time complexity.

Examples

Example 1:

Input:

["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]

Output:

[null, true, false, true, 2, true, false, 2]

Explanation:

RandomizedSet randomizedSet = new RandomizedSet();
randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomizedSet.remove(2); // Returns false as 2 does not exist in the set.
randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].
randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.
randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].
randomizedSet.insert(2); // 2 was already in the set, so return false.
randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.

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Solution 💡

Use a hash map to store the elements and their indices in an array. This allows O(1) time complexity for insertion, deletion, and getting a random element.

Java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Random;

class RandomizedSet {
    private HashMap<Integer, Integer> map;
    private ArrayList<Integer> list;
    private Random rand;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
        rand = new Random();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) return false;
        map.put(val, list.size());
        list.add(val);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        int index = map.get(val);
        int lastElement = list.get(list.size() - 1);
        list.set(index, lastElement);
        map.put(lastElement, index);
        list.remove(list.size() - 1);
        map.remove(val);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(rand.nextInt(list.size()));
    }
}

Time Complexity ⏳

• insert operation: O(1)

• remove operation: O(1)

• getRandom operation: O(1)

Space Complexity 💾

• The space complexity is O(n), where n is the number of elements in the set, due to the storage in the hash map and the list.

You can find the full Solution.java file here.